1. Field of the Invention
The present invention relates to a method, apparatus and program for creating a threshold value matrix wherein the threshold value matrix used in quantization of the n-value signal in each pixel to an m-value (n>m>1) is created by a computation device using dither process.
2. Description of Related Art
The image outputting device is exemplified by a printer using an inkjet method or electrophotographic process, and an image output device using the area modulation such as a liquid crystal display. In such an image output device, what is called half-tone processing is applied to an input image, and a gray scale image is converted into the area-modulated image. This process is typically represented by the “error diffusion method” and dither process”.
The principle of the error diffusion method is introduced in R. W. Floyd and L. Steinberg “An Adaptive Algorithm for Spatial Gray Scale”, SID 75 Digest (1976). As described in this document, a step is taken to compute the difference in density (error data) between the density of each pixel of interest of input image, and the output pixel density (dot on/off status) outputted from a recording apparatus after halftone processing. Halftone processing is carried out while the error data as a result of computation is distributed wherein a specific weight is assigned to the peripheral pixel before halftone processing.
In the dither process, by contrast, comparison is made between the density of each pixel of interest of input image and the threshold value corresponding thereto, and a decision is made to see whether or not the dot of the pixel of interest is turned on. The threshold value is held as the matrix, and the dot pattern produced by the dither process is determined by the threshold value matrix.
As will apparent from the above, halftone processing speed is higher in the dither process than in the error diffusion method However, the error diffusion method provides more uniform dot distribution, hence a high-quality image.
In one of the techniques disclosed in recent years, the pattern of the threshold value matrix used in the dither process is improved to get the dot pattern as that obtained from the error diffusion method. A proposal of the implemented device is found in the U.S. Pat. No. 5,111,310 (Patent Document 1). The Specification of this patent discloses the quantization art of implementing the error diffusion-like halftone reproduction wherein, while the high-speed processing on the equal level as that of the conventional dither process in terms of halftone processing is maintained by using a sufficiently large threshold value matrix, the matrix is assigned with the pattern showing the space frequency characteristic known under the name of blue noise, whereby error diffusion-like halftone reproduction is achieved (hereinafter referred to as “blue noise dither process”).
The details of the blue noise dither process are disclosed in a great number of documents including “Digital Halftoning” Robert Ulichney (The MIT Press Cambridge, Massachusetts London, England). The blue noise dither process is the halftone technique using the threshold value matrix wherein the noise of the output image is reduced by suppressing the power spectrum of the low-frequency component highly sensitive to the human eyesight.
Several other techniques are disclosed in the following Patent Documents:
[Patent Document 1] U.S. Pat. No. 5,111,310 (FIG. 1, P1)
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 2001-28685 (Tokkai) (FIG. 1, P1)
[Patent Document 3] Japanese Unexamined Patent Application Publication No. 2001-111829 (Tokkai) (FIG. 1, P1)
The Patent Document 1 is the basic patent of the blue noise mask. The image re-created through a bypass filter after the FFT operation of the random pattern is compared with the original image, and the pixel of greater error is reversed.
The Patent Document 2 shows the method of reducing the threshold value matrix creation process. The area is divided into small areas, and a halftone matrix is formed in each small area. Then combination is made in the final phase.
Further, the Patent Document 3 describes the method wherein setting is made in such a way that the initial dot pattern is formed in a regular triangle, to ensure that a highlight texture will not appear when a mask is created according to the blue noise mask process. The dot arrangement is also determined by using the space potential.
However, the blue noise dither process requires power spectrum to be computed many times for each gradation value. When the threshold value matrix is larger, much time is required to create the matrix. This problem has been left unsolved.
Further, in the blue noise dither process, there is a gradation area wherein a specific pattern such as a chequered pattern appears and the image quality is degraded.